Ethanol and other high heat of vaporization (HoV) fuels result in substantial cooling of the fresh charge, especially in direct injection (DI) engines. The effect of charge cooling combined with the inherent high chemical octane of ethanol make it a very knock resistant fuel. Currently, the knock resistance of a fuel is characterized by the Research Octane Number (RON) and the Motor Octane Number (MON). However, the RON and MON tests use carburetion for fuel metering and thus likely do not replicate the effect of charge cooling for DI engines. The operating conditions of the RON and MON tests also do not replicate the very retarded combustion phasing encountered with modern boosted DI engines operating at low-speed high-load.

In this study, the knock resistance of a matrix of ethanol-gasoline blends was determined in a state-of-the-art single cylinder engine equipped with three separate fuel systems: upstream, pre-vaporized fuel injection (UFI); port fuel injection (PFI); and DI. Constant inlet temperature was held downstream of the injector for UFI and upstream of the injectors for PFI and DI. For each fuel, engine inlet pressure was swept at borderline knocking conditions at constant engine speed using each of the three fuel systems. This test method characterized each fuel's knocking behavior over a wide range of conditions, including those typical of boosted DI engines.

Comparison of UFI and DI results allowed the chemical octane effect on knock to be separated from the evaporative charge cooling effect. These effects were found to be of comparable importance for ethanol blends.

An outcome of the test method was the discovery of an interaction between combustion phasing and the sensitivity of a fuel's autoignition kinetics to temperature. For a given gasoline blendstock, increasing ethanol content significantly increased knock-limited performance with combustion phasing near the thermodynamic optimum, as expected. However, due to ethanol's high sensitivity, knock-limited performance improved to a much greater extent with increasing ethanol content as combustion phasing was retarded. This effect was further enhanced by charge cooling with DI. Increasing ethanol content also significantly increased the knock-limited performance before enrichment was required to control exhaust gas temperature.

The RON ratings of the fuels did not fully reflect the observed knock resistance of mid-to-high level ethanol blends (E20 and higher). K, the weighting factor for MON in the Octane Index, decreased with increasing combustion phasing retard and with increasing evaporative charge cooling, and increased with increasing inlet temperature and increasing compression ratio.